The invention described herein relates to nuclear reactors and more particularly to a process for dissolving into coolant normally circulated through the reactor, corrosion from the reactor internal surfaces which contain radioactive products.
During operation of a nuclear reactor, the fission process occurring in reactor fuel generates radioactive fission gases and radioactive fission products such as iodine 131 and 133, cesium 134 and 137, molybdenum 99, xenon 133 and activates reactor structural materials, such as nickel to form cobalt 58, and the like, which desirably must be removed from the coolant before reactor refueling can take place. As the reactor coolant temperature and pressure are reduced in preparation for refueling, these fission gases and products are released to the coolant and such release terminates soon after the cool-down procedure has been completed. The reactor coolant system must then be purged of fission gases before removing the reactor closure head to preclude the possibility of fission gas release to the atmosphere. Likewise, fission product removal is necessary to minimize contamination of the reactor cavity water and the associated system components. Since standard procedures are followed to effect such gas and fission product removal, techniques have been established for capturing the fission gases, and the ionic fission products are adequately removed by known ion exchange purification processes.
After the coolant is depressurized, reduced in temperature and oxygen added to the coolant water, cobalt 58, which is generated by activation of nickel in a high radiation field during reactor operation, is released from the internal surfaces of the reactor, and rapidly dissolves in the cold oxygenated cooling water. This high intensity radioactive isotope makes refueling time-consuming because protective measures must be taken to effect its removal. It must therefore be reduced to a very low level prior to actual commencement of the fuel transfer operations.
Unlike some other species of radioactivity, cobalt 58 is released into the coolant most readily when the water is cool and contains a small amount of oxygen. The methods currently used to oxygenate coolant in a closed reactor involves reducing the hydrogen level therein to about 4 cc per kg, or less, and draining the reactor to about 1/3 of full volume while charging nitrogen into the space left void by the withdrawn coolant. Air is then pumped through the void space to transfer as much oxygen as possible to the water coolant. This action transfers oxygen into the water to achieve a reasonable degree of oxygenation which then causes the nickel and cobalt to dissolve in the solution.
The primary disadvantage of this method is that approximately twenty-four hours are required to obtain adequate oxygenation because the air contacts only a small portion of the coolant during the oxygenation process and circulation of the coolant throughout the system is not possible once partial draining has taken place. This time period is significant, particularly when the process is carried out on those reactors designed to a refueling schedule of 7 days or less. Coolant oxygenation and removal of cobalt 58, nickel and other radioactive species, consume approximately 25% of the time allotted for reactor refueling. It therefore is apparent that time reduction in the radioactive species removal process will significantly affect reactor down time which in turn helps minimize the electric utility's costs, and further, can provide the potential for increased revenue which flows to the utility when the reactor is in operation.
Alternatively, the injection of air or pure oxygen directly into the reactor coolant system for oxygenation purposes rather than flowing it across the water, may be acceptable for long time and relatively relaxed refueling schedules since a longer time period is required to achieve the desired degree of oxygenation. However, bubbles may appear under the reactor head or in pumps or other apparatus and form air locks which are objectionable from an operating standpoint. Since hydrogen is present in the system, the introduction of gaseous oxygen may also present a dangerous combustible mixture when vented to the atmosphere and then accidentally exposing the combined mixture to ignitable conditions.